The airport controller authorized the use of runway 25 for the departure of the Cessna 421C. However, he did not indicate at what point the runway should be made active or ensure the communication of that fact to the ground controller by posting the runway strip header in the flight data strip bay as required by Operations Letter 97-11. The practice of the ground controller sometimes initiating the transfer of responsibility for the runway and posting the runway strip header offers flexibility in busy situations. However, it also adds uncertainty as to who will do which actions and increases workload by introducing a requirement for additional communication. In this occurrence, both controllers assumed that the other would take action to confirm ownership of the runway. Neither controller performed the required activity. The coordination and teamwork required by Operations Letter 91-4 did not take place during this occurrence. The specific behaviour of the controllers associated with the unsafe issuance of take-off clearance cannot with confidence be tied to behaviours associated with fatigue. The failure to post a runway status strip is attributable more to procedural uncertainty, as to who was responsible for the designation of the active runway, than to the airport controller's subjective feeling of fatigue. The airport controller's authorization of the Cessna 421C to taxi to position on runway 25 did not serve as a signal that the runway was or should have been made active. There is no indication that either controller sought to clarify the status of the runway or of the taxiway that crossed it. The mandatory scan of runway 25 prior to issuing take-off clearance would probably not have shown the Piper Seminole in proximity to the runway because that aircraft did not enter runway 25 until 48 seconds later. At the time that the Cessna 421C was issued take-off clearance, the Piper Seminole was likely still on taxiway A or B. The absence of the third controller, who was on a relief break, eliminated the third set of eyes whose duty it was to act as a spotter for the airport controller during the heavy traffic situation being experienced at the time. The controllers on duty did not scan the manoeuvring surfaces in sufficient detail or with sufficient regularity to detect the approaching runway incursion. The method of communications with Toronto ACC may have hindered the normal internal communications process between the ground and the airport controller. The fact that the ground controller, rather than the airport controller, received the clearance validation message from Toronto may have eliminated one of the personal defences normally exercised by the airport controller. In the past, when the airport controller received clearance validation messages from the ACC, he had been in the habit of confirming ownership of the departure runway if it was other than the one currently being used. It may be that, because he did not receive the clearance validation, which was the first part of the personal defence process, he did not follow on with the second part of the process, the confirmation of runway ownership.Analysis The airport controller authorized the use of runway 25 for the departure of the Cessna 421C. However, he did not indicate at what point the runway should be made active or ensure the communication of that fact to the ground controller by posting the runway strip header in the flight data strip bay as required by Operations Letter 97-11. The practice of the ground controller sometimes initiating the transfer of responsibility for the runway and posting the runway strip header offers flexibility in busy situations. However, it also adds uncertainty as to who will do which actions and increases workload by introducing a requirement for additional communication. In this occurrence, both controllers assumed that the other would take action to confirm ownership of the runway. Neither controller performed the required activity. The coordination and teamwork required by Operations Letter 91-4 did not take place during this occurrence. The specific behaviour of the controllers associated with the unsafe issuance of take-off clearance cannot with confidence be tied to behaviours associated with fatigue. The failure to post a runway status strip is attributable more to procedural uncertainty, as to who was responsible for the designation of the active runway, than to the airport controller's subjective feeling of fatigue. The airport controller's authorization of the Cessna 421C to taxi to position on runway 25 did not serve as a signal that the runway was or should have been made active. There is no indication that either controller sought to clarify the status of the runway or of the taxiway that crossed it. The mandatory scan of runway 25 prior to issuing take-off clearance would probably not have shown the Piper Seminole in proximity to the runway because that aircraft did not enter runway 25 until 48 seconds later. At the time that the Cessna 421C was issued take-off clearance, the Piper Seminole was likely still on taxiway A or B. The absence of the third controller, who was on a relief break, eliminated the third set of eyes whose duty it was to act as a spotter for the airport controller during the heavy traffic situation being experienced at the time. The controllers on duty did not scan the manoeuvring surfaces in sufficient detail or with sufficient regularity to detect the approaching runway incursion. The method of communications with Toronto ACC may have hindered the normal internal communications process between the ground and the airport controller. The fact that the ground controller, rather than the airport controller, received the clearance validation message from Toronto may have eliminated one of the personal defences normally exercised by the airport controller. In the past, when the airport controller received clearance validation messages from the ACC, he had been in the habit of confirming ownership of the departure runway if it was other than the one currently being used. It may be that, because he did not receive the clearance validation, which was the first part of the personal defence process, he did not follow on with the second part of the process, the confirmation of runway ownership. The controllers involved in the occurrence were qualified and current for their positions. The need to use the hot line for communications with the Satellite sector radar controller imposed an added coordination requirement on the ground and airport controllers in that clearance validation information, normally routed directly to the airport controller, had to pass through the ground controller. Local operations letters specify that the active runway must be indicated by posting a runway strip header, and also specify who is responsible for doing so, but do not detail when the runway should be made active. Local practices varied concerning who would initiate the control transfer of an alternate active runway and who would post the runway strip header. There was no overt declaration of the activation of runway 25 by the airport controller, no signal posted to indicate that the status of the runway had changed, and no coordination to ensure mutual awareness of the situation. The airport controller assumed that runway 25 had been transferred to the control of the airport controller when the ground controller advised that the Cessna 421C was released. The ground controller assumed that, since the airport controller had not actively taken control of runway 25, it was still under the control of ground control. The defence provided by the lookout duties of the third controller during heavy traffic periods was eliminated by that controller's need for a break and the absence of a replacement. Staffing met unit standards. The controllers on duty did not scan the manoeuvring surfaces in sufficient detail or with sufficient regularity to detect the approaching runway incursion.Findings The controllers involved in the occurrence were qualified and current for their positions. The need to use the hot line for communications with the Satellite sector radar controller imposed an added coordination requirement on the ground and airport controllers in that clearance validation information, normally routed directly to the airport controller, had to pass through the ground controller. Local operations letters specify that the active runway must be indicated by posting a runway strip header, and also specify who is responsible for doing so, but do not detail when the runway should be made active. Local practices varied concerning who would initiate the control transfer of an alternate active runway and who would post the runway strip header. There was no overt declaration of the activation of runway 25 by the airport controller, no signal posted to indicate that the status of the runway had changed, and no coordination to ensure mutual awareness of the situation. The airport controller assumed that runway 25 had been transferred to the control of the airport controller when the ground controller advised that the Cessna 421C was released. The ground controller assumed that, since the airport controller had not actively taken control of runway 25, it was still under the control of ground control. The defence provided by the lookout duties of the third controller during heavy traffic periods was eliminated by that controller's need for a break and the absence of a replacement. Staffing met unit standards. The controllers on duty did not scan the manoeuvring surfaces in sufficient detail or with sufficient regularity to detect the approaching runway incursion. The airport controller cleared the Cessna 421C to take off on runway 25 when the Piper Seminole was cleared to cross the runway, without first actively designating it as an active runway and without taking action to confirm that the ground controller was aware that the runway was active. Contributing to the occurrence were the inconsistent application of a standard procedure for the timing, designation, and posting of temporary active runway information and the lack of a sufficiently detailed scan of the manoeuvring surfaces.Causes and Contributing Factors The airport controller cleared the Cessna 421C to take off on runway 25 when the Piper Seminole was cleared to cross the runway, without first actively designating it as an active runway and without taking action to confirm that the ground controller was aware that the runway was active. Contributing to the occurrence were the inconsistent application of a standard procedure for the timing, designation, and posting of temporary active runway information and the lack of a sufficiently detailed scan of the manoeuvring surfaces. Kitchener-Waterloo control tower Operations Letter 97-11 is now supplemented by Operations Letter 99-6 which clarifies that both controllers have responsibility to ensure that runway status header strips are in place. If the ground controller transfers control of a runway by assigning it to a departing aircraft, then the ground controller must ensure that the runway status display indicates that the airport controller has jurisdiction of the runway and the ground controller no longer has approval to use it.Safety Action Kitchener-Waterloo control tower Operations Letter 97-11 is now supplemented by Operations Letter 99-6 which clarifies that both controllers have responsibility to ensure that runway status header strips are in place. If the ground controller transfers control of a runway by assigning it to a departing aircraft, then the ground controller must ensure that the runway status display indicates that the airport controller has jurisdiction of the runway and the ground controller no longer has approval to use it.